Heteropolyacids are well known to be useful as acid catalysts or oxidation catalysts. A heteropolyacid and/or heteropolyacid salt supported on a support can prevent the elution of the heteropolyacid or the precipitation of carbonaceous material or increase the surface area to elevate the efficiency in a reaction with a substrate having low affinity for the heteropolyacid. Therefore, the catalyst using a heteropolyacid and/or a heteropolyacid salt as the effective component is generally used as a supported catalyst where the heteropolyacid and/or heteropolyacid salt is supported on a porous support such as silica.
The supported state of the catalyst component in the supported catalyst includes several types. Specific examples thereof include a uniform type, an egg shell type (skin type) and an egg white type. The uniform type, egg shell type and egg white type as used herein each means a supported catalyst in the following state (see, Yoshio ONO et al., Shokubai no Jiten (Dictionary of Catalyst), 1st ed., 1st imp. pages 102, 108 and 585, Asakura Shoten (Nov. 1, 2000)).
Uniform Type:
So-called uniform distribution or uniform loading.
Egg Shell Type:
This is one of active component distribution states in a support particle or molded body of a supported catalyst and means a state where the active component is present only on the outer surface of the support particle or molded body.
Egg White Type:
This is one of active component distribution states in a support particle or molded body of a supported catalyst and means a state where the active component is present in an inner layer of the support.
The heteropolyacid and/or heteropolyacid salt is used as a supported catalyst, for example, in the hydration of an olefin (see, Japanese Unexamined Patent Publication No. 11-322646 (JP-A-11-322646)) the production of a carboxylic acid ester (see, Japanese Unexamined Patent Publication Nos. 11-263748 and 9-118647 (JP-A-11-263748 and JP-A-9-118647)) and the production of an acetic acid by ethylene oxidation (see, Japanese Unexamined Patent Publication No. 7-89896 (JP-A-7-89896)).
Heretofore, the heteropolyacid and/or heteropolyacid salt has been generally supported in the uniform type. However, the uniform type has a problem that, particularly in a surface-type reaction using the diffusion of raw material as a rate-determining factor, the heteropolyacid and/or heteropolyacid salt supported inside the support may not efficiently participate in the reaction.
On the other hand, as for the method for loading a heteropolyacid in the state other than the uniform type, U.S. Pat. No. 5,919,725 discloses a method of loading a heteropolyacid salt in a specific position slightly inside the surface (so-called egg white type). However, the heteropolyacid salt in the egg white-type catalyst is present inside the support and, in a surface-type reaction of using the diffusion of raw material as a rate-determining factor, cannot make efficient contact with a reactant.
To solve this problem, a catalyst where the heteropolyacid and/or heteropolyacid salt is supported on the surface (so-called egg shell type or skin type) is considered to be more effective.
Particularly, in an oxidation reaction which is a surface-type reaction, it is important to load the catalyst component in the vicinity of surface. For example, JP-A-7-89896 discloses a method of synthesizing an acetic acid from ethylene and oxygen in the presence of a catalyst containing (a) metal palladium and (b) at least one compound selected from heteropolyacids and salts thereof. According to this method, the palladium metal interacts with the heteropolyacid and/or heteropolyacid salt to exert very high activity and selectivity and thereby exhibit excellent activity and selectivity for the production of an acetic acid.
In this example, it is presumed that the metal palladium is supported in the egg shell-type state and the heteropolyacid and/or heteropolyacid salt is, in view of its general loading method on a support, supported in the uniform type. Also in this case, the use efficiency is considered to be elevated by loading the heteropolyacid and/or heteropolyacid salt in the egg shell-type state. Furthermore, when both the metal palladium and the heteropolyacid and/or heteropolyacid salt are supported in the egg shell-type state, these two members can be made present very adjacently and this seems to bring about increase in the probability of interaction and, in turn, production of a higher performance catalyst, In addition, by loading the heteropolyacid and/or heteropolyacid salt in the egg shell-type state, the absolute amount of heteropolyacid and/or heteropolyacid salt can be decreased as compared with the uniform type and this is advantageous in view of profitability and reduction in the cost for recovery and reproduction.
However, a catalyst where a heteropolyacid and/or a heteropolyacid salt is supported in the egg shell-type state, and a production process of the catalyst have been heretofore unknown.